1. Field of the Invention
The present invention refers to an integrated device in emitter-switching configuration and related manufacturing process.
2. Description of the Related Art
Among the different integrated devices in emitter-switching configuration there is a particular type that is constituted by a high voltage power bipolar transistor and by a low voltage signal transistor where the emitter terminal of the power transistor is connected with the charge collection terminal (drain or collector) of the low voltage transistor. Generally the power transistor has a Breakdown voltage that can exceed 1 kV and it must be able to commutate at high frequencies (at about 200 khz). The low voltage transistor has a Breakdown voltage lower than 60V and it has fast commutation features that assure the use of the emitter switching device at high frequencies.
An example of a device in emitter-switching configuration is shown in FIG. 1. Such a device comprises a high voltage bipolar transistor Th and a MOS or bipolar low voltage device, indicated schematically by a block 1. The emitter terminal of the transistor Th is connected with the charge collection terminal (drain or collector) of the low voltage transistor 1. The base terminal Bh and the terminal C (base terminal or gate terminal of the transistor 1) constitute the control terminals of the device, which allow the connection between two circuit parts connected with the terminal Ch (collector terminal of the transistor Th) and Ee (emitter or source terminal of the transistor 1) which can be opened and closed alternately. Generally the terminal Ee is connected with ground, the terminal Ch is connected with a first terminal of a load L the second terminal of which is connected with a supply voltage Vcc. The device comprises a quenching element B constituted, for example, by a Zener diode or by a series of diodes which is connected between the terminals Bh and Ee.
During the turning on of the device, the quenching time of the transistor Th is greater than the quenching time of the transistor 1 because of the large accumulation of charges in the base of the transistor Th. After the transistor 1 is quenched, bringing the emitter current of the transistor Th to zero, a current flows from the collector of the transistor Th through its base and this current is discharged to ground through the quenching element B. Once all the charge residuals in the base of the transistor Th have been eliminated, its collector current, and hence the current in the load L, is brought to zero.
In known devices in emitter-switching configuration, the quenching element is formed, for example, by discrete components suitably connected with the two transistors; this solution increases the production cost and the size of the device.
In the case wherein both the high voltage transistor and the low voltage transistor are integrated in the same semiconductor chip, the quenching element is normally formed within an insulated well of the region wherein the control circuits are allocated.
A different solution is disclosed in U.S. Pat. No. 6,127,723 where an integrated device in emitter-switching configuration is disclosed comprising a high voltage bipolar transistor and a low voltage transistor and wherein the quenching element, a Zener diode, is formed in the base region or in the emitter region of the high voltage bipolar transistor. In this way the integrated device occupies a limited space in the semiconductor chip and a low resistance in series with the quenching element is assured so that the power dissipation at the quenching stage is reduced.
However this solution presents the disadvantage of reducing the whole area of the high voltage bipolar transistor and of the low voltage transistor in order to form the quenching element. In the case wherein the Zener diode is formed in the emitter region of the high voltage transistor, besides the reduction of the whole area of the low voltage transistor a reduction of the efficiency of the high voltage transistor in the zone underlying the Zener diode is achieved.